Table tops are easy; any old door or slab will do. Legs are tougher; you can go to IKEA or Mockett and buy any number of different designs of legs that can be screwed on, but nothing so simple to install as Phillipe Nigro's idea for a "universal base" that just clamps on to the top.
Having had many a table leg rip out of a veneered slab door used as a table top, or had them slip off an IKEA sawhorse and crash to the floor, I would call this a giant leap for mankind if it is priced right, like really, really cheap.
Design Available for Production via PSFK
More nice legs in TreeHugger:
Roy Tam Ash Table/Desk
Clevr: Marta Antoszkiewicz's Kitchenette, for Urban Living